Measurement of the Anisotropy Power Spectrum of the Radio Synchrotron Background

TL;DR

This study measures the anisotropy power spectrum of the radio synchrotron background at 140 MHz, revealing higher-than-expected fluctuations that suggest a vast, clustered population of faint sources.

Contribution

First targeted measurement of radio synchrotron background anisotropies at 140 MHz, providing new insights into its origin and source distribution.

Findings

Anisotropy power exceeds that from known point sources above 100 micro-Jy.

Results imply a highly numerous, faint, and possibly clustered source population.

Measurement covers scales from 2 degrees to 0.2 arcminutes.

Abstract

We present the first targeted measurement of the power spectrum of anisotropies of the radio synchrotron background, at 140 MHz where it is the overwhelmingly dominant photon background. This measurement is important for understanding the background level of radio sky brightness, which is dominated by steep-spectrum synchrotron radiation at frequencies below 0.5 GHz and has been measured to be significantly higher than that which can be produced by known classes of extragalactic sources and most models of Galactic halo emission. We determine the anisotropy power spectrum on scales ranging from 2 degrees to 0.2 arcminutes with LOFAR observations of two 18 square degree fields -- one centered on the Northern hemisphere coldest patch of radio sky where the Galactic contribution is smallest and one offset from that location by 15 degrees. We find that the anisotropy power is higher than that attributable to the distribution of point sources above 100 micro-Jy in flux. This level of radio anisotropy power indicates that if it results from point sources, those sources are likely at low fluxes and incredibly numerous, and likely clustered in a specific manner.